Baseband Processor For Mixed Mode Land Mobile Radio
CMX880
Transmitting C4FM and CQPSK Signals
The CMX880 generates filtered symbol levels at the 4800 symbols/s symbol rate in accordance with the APCO
25 Recommended Common Air Interface, TSB102.BAAA. The frame sync pattern insertion, forward error
correction and other data encoding and message formatting, must be performed by the external DSP, before
symbols are transferred to the CMX880.
The symbol codes of the symbols to be transmitted are transferred to the CMX880 over the FSB. The format of
the symbol word transferred over the FSB and symbol levels are defined in Table 13. A single 16-bit word buffer
enables 8 symbol codes (2 octets) to be transferred from the DSP to the CMX880 by a single FSB transaction.
The DSP must be able to provide successive symbol words at 600words/s to ensure the symbol rate of 4800
symbols/s is maintained.
Table 13 C4FM Symbol Word Encoding
msb
15
lsb
0
Bits:
14 13 12 11 10
9
8
7
6
5
4
3
2
1
Symbol
7 (first)
6
5
4
3
2
1
0 (last)
Bit 1 Bit 0 Bit 1 Bit 0 Bit 1 Bit 0 Bit 1 Bit 0 Bit 1 Bit 0 Bit 1 Bit 0 Bit 1 Bit 0 Bit 1 Bit 0
2-bit symbol encoding
Bit 1
Bit 0
Symbol
C4FM Freq.
Deviation
+1.8kHz
+0.6kHz
-0.6kHz
CQPSK
Phase
+135 deg.
+45 deg.
-45 deg.
-135 deg.
0
0
1
1
1
0
0
1
+3
+1
-1
-3
-1.8kHz
For C4FM transmitted signals the symbol levels are decoded and passed as scaled impulses to the input of a
Raised Cosine, H(f) and Shaping, P(f) filter, D-to-A converted, then output to the FM Modulator as shown in
Figure 15. The symbol levels need to be defined prior to the CMX880 transmitting a C4FM signal. The symbol
levels are defined by writing the +1 unit symbol level into the ‘TX C4FM/CQPSK UNIT LEVEL’ word of the
‘PROGRAMMING REGISTER’ ($C8). The CMX880 will formulate the other two symbol levels from this data
before transmission.
CMX880
Filtered
baseband
signal
4-level
symbols
4-Level
Symbol
Impulses
Raised
Cosine, H(f)
and
Symbol Code
to Impulse
Level
FSB
D to A
Converter
Frequency
Modulator
Shaping, P(f)
Conversion.
Filter
Figure 15 C4FM Transmitter Processing Path
The RC filter transfer characteristic has a flat group delay over the pass band for |f|<2880Hz, with a magnitude
response of approximately:
|H(f)| = 1
for |f|<1920Hz;
|H(f)| = 0.5 + 0.5cos(2pf/1920)
|H(f)| = 0
for 1920Hz<|f|<2880Hz;
for |f|>2880Hz.
ã 2001 Consumer Microcircuits Limited
33
D/880/1
CMLMICRO [ CML MICROCIRCUITS ]
